Refrigerators, such as those employed to store food or the like in households, typically employ an evaporator unit in which refrigerant fluid at low pressure is evaporated to cool a refrigerating compartment. The fluid is then compressed and delivered to a condenser where heat is extracted from the fluid. These evaporator units typically employ a refrigerant delivery tube arranged in a plurality of coil turns arranged in two or more adjacent rows. A plurality of spaced apart, heat exchanging fins are connected to the coil turns and extend transversely between adjacent rows of the coil in order to conduct heat from the refrigerating compartment to the coil, and thus to the refrigerant fluid.
Condensation normally forms on both the coil and the fins of these evaporator units that results in an accumulation of frost and ice which eventually impair the operating efficiency of the unit. Consequently, it is necessary to periodically remove the frost and ice by turning off the unit and/or applying localized heat to the areas of frost and ice. So called "frost free" refrigerators eliminate the need for shutting down the unit in order to defrost the evaporator unit by mounting a heating element adjacent the rows of coil turns. The heating element comprises a resistance wire disposed within elongate rod having a sheathing material such as Inconel for withstanding relatively high temperatures. The rod is mounted by means of chips and/or fittings on the coil or fins so as to be free-floating between coil turns, adjacent the bottom of the unit. The heating element is periodically energized in order to convert the frost and ice to liquid which then falls into a drip pan positioned beneath the unit. The heating element also functions to melt ice forming within the drip pan itself.
Evaporator units of the type described above were less than completely satisfactory in several respects. The method of mounting the heating element on the unit was relatively costly due to the special mounting hardware, as well as the labor necessary to assemble the hardware. Heat transfer from the heating element to the coils and fins was somewhat inefficient since the mounting hardware insulated the heating element from direct contact with the coils and fins; consequently, a certain amount of heat energy was transferred from the heating element to the surrounding environment which could otherwise be directly transferred to the coils and fins by conduction if the heating element was mounted in direct engagement with the coils and/or fins. Because of the inefficiency mentioned above, incomplete defrosting of the coils and fins was less than optimum, particularly near the bottom of the unit.
Finally, the sheathing materials employed in the heating rod were either relatively thick or were comprised of special alloys selected to withstand relatively high surface temperatures because of the fact that the rod was effectively insulated from the remainder of the unit by the mounting hardware. Thinner or less costly sheathing materials could be employed if a mounting arrangement were provided which reduced the surface temperature of the heating rod.